The present invention relates to a digital reproduced signal processing apparatus and particularly, to a digital reproduced signal processing apparatus which employs partial response suited for use in a digital reproduced signal processing circuit provided in a playback system for reproduction of digital records based on code words where the minimum code length is relatively extensive.
The attention has recently been focused on digital recording media including compact disks (referred to as CDs hereinafter) and digital versatile disks (referred to as DVDs hereinafter) which can save records almost semipermanently.
For playing back signals recorded on CDs and DVDs, a variety of analog reproduced signal processing apparatuses are commonly used because their circuit size is relatively simple and small and thus can be fabricated at less cost.
As a first conventional example, an analog DVD reproduced signal processing apparatus is now explained referring to FIG. 7.
FIG. 7 is a block diagram 2 showing an arrangement of a conventional analog reproduced signal processing apparatus Z, where a reproduced signal runs as is explained below.
As shown in FIG. 7, the analog reproduced signal is read out from a recording medium 101 with a read head 102 and fed to an analog filter 103. The filter 103 removes high frequency noise components in the signal and also emphasizes a particular frequency range of the signal to minimize jitters.
The signal filtered by the filter 103 is then transferred to a DC level control circuit 104 and a level comparator 105.
The DC level control circuit 104 extracts a DC component from the filtered analog reproduced signal and releases it to the comparator 105 where it is used to determine a slice level.
The comparator 105 compares the filtered analog reproduced signal with the slice level to judge whether it is greater or smaller than the level and its binary judgment is released as a binary data.
The binary data is fed to a phase comparator 106 where it is compared in phase with a clock signal delivered from a voltage-controlled oscillator 108. A phase error signal resulting from an error in the phase comparison is transmitted via a loop filter 107 to the voltage-controlled oscillator 108 where it is used for controlling the oscillation on the clock signal.
While a procedure of signal processing in the analog reproduced signal processing apparatus Z is carried out as described above, the reproduced signal read out from CD or DVD records has a fair but not satisfied level. It is hence desired to develop improved reproduced signal processing apparatuses for processing the signal at a higher level.
The DVD system uses signals based on xe2x80x9ceight to fourteen modulationxe2x80x9d (referred to as EFM hereinafter) codes or EFM-Plus codes where the minimum code length of a code word is 3T. The frequency response in an optical section of the DVD system is as shown in FIG. 6.
In general, DVD records are at high density and the channel rate or reproduction rate of their reproduced signals is set to a higher frequency than that of the frequency response in the optical system shown in FIG. 6. With the frequency response in the optical system, any 1T signal in the code word may hence be reproduced at a significantly attenuated level. More specifically, the setting of the minimum code length to 3T, for example, in FEM permits the minimum pit length to be identified of as large as 3T. If a pit having a length of 1T is contained in a code length of the reproduced rate, its signal size is too small to be identified or picked up, i.e. the 1T signal component is attenuated to a hardly reproduced level.
It is therefore said that the S/N (signal to noise) ratio in the reproduction section of the DVD system is considerably high. Using such a high S/N ratio and the EFM signals of which the minimum code length is limited, the DVD system provides a capability of high density recording.
For magnetic disk systems, a unique reproduced signal processing method called xe2x80x9cpartial response maximum likelihoodxe2x80x9d (referred to as PRML hereinafter) has been employed to increase the recording density.
The PRML method is a combination of a partial response technique in the communications technology and a maximum likelihood decoding technique in the code technology. It is hence necessary to select an optimum type of the partial response of which the frequency response matches that of a reproduced signal of RPML.
For example, the reproduced signal from a magnetic disk has a band-pass characteristic such as the magnetic recording characteristic shown in FIG. 8. The partial response of which the frequency response matches that of the reproduced signal of the magnetic recording representing the band-pass characteristic may be characterized by (1, 0, xe2x88x921). As shown in FIG. 8, the frequency response of the reproduced signal of the magnetic recording is very similar to that of the partial response (1, 0, xe2x88x921) and they may easily be equalized without emphasizing a higher frequency range. In particular, the frequency characteristic of the partial response is closely related to the code words.
The application of the PRML method to the DVD signals has been attempted for increasing the recording density on a DVD medium with the help of the advantage of the PRML method. It is hence desired for processing the DVD signals of the PRML method to develop an improved digital reproduced signal processing apparatus which has a higher capability of reading the signals.
FIG. 9 is a block diagram showing the arrangement of a digital reproduced signal processing apparatus, as a second prior art, for magnet optical disks on which signals of the PRML method are recorded. A procedure of processing a reproduced signal in the signal processing apparatus Y is explained below.
As shown in FIG. 9, the analog reproduced signal read by a read head 202 is fed to an analog filter 203 where its high frequency noises are cut off and its signal components at a specific range are controlled to be close to the frequency characteristic of the partial response (1, 1). The analog reproduced signal filtered is then transmitted to a DC level control circuit 204, a level comparator 205, and an analog/digital converter 209.
The DC level control circuit 204 extracts a DC component from the analog reproduced signal and transfers it to the level comparator 205 where its is used as a slice level. The comparator 205 compares the filtered analog reproduced signal with the slice level and releases its comparison signal to a phase comparator 206.
The phase comparator 206 compares the comparison signal with a clock signal supplied from a voltage-controller oscillator 208 and transmits its phase error signal via a loop filter 207 to the voltage-controlled oscillator 208. The clock signal is also fed to the analog/digital converter 209 and a Viterbi decoder 210.
The filtered analog reproduced signal is converted by the analog/digital converter 209 to a digital signal which is then sent to the Viterbi decoder 210 of the partial response (1, 1) type where it is converted to a binary data
FIG. 10 is a frequency response diagram showing a frequency response of the reproduced signal from an magnet optical disk and a partial response (1, 1). As apparent from that figure, the two characteristic curves are similar in shape indicating that the frequency response is good without emphasizing a high frequency range in the equalization.
The digital reproduced signal processing apparatus Y for magnet optical disks carrying the PRML signals allows the signals to be reproduced from their recording medium at a higher fidelity than the analog reproduced signal processing apparatus Z of the first prior art.
However, the digital reproduced signal processing apparatus Y using the partial response (1, 1) still has some disadvantageous features which are hardly compatible with the DVD or CD system.
As described previously, the DVD or CD system employs commonly EFM codes or EFM-Plus codes of which the minimum code length is 3T. The EFM or EFM-Plus codes however carry less information at a high frequency range. Accordingly, while the bit rate for recording in the DVD or CD system is set relatively high, the frequency response of reproduced signals may exhibit a significantly attenuated level at its high frequency range. This causes high-frequency noises in the reproduced signal of EFM or EFM-Plus codes of the DVD or CD system to be emphasized in the digital signal processing using the partial response (1, 1) of the frequency response, thus inhibiting the reading of the reproduced signal at a high fidelity.
Since increasing the order of the partial response declines the effect of emphasizing a high frequency range in the frequency response, a higher order of the partial response may be used to eliminate a problem of emphasizing the high frequency noises in the digital reproduced signal processing apparatus. However, the higher the order of the partial response, the more the circuitry arrangement of the apparatus will be complicated.
FIG. 2 illustrates a frequency response of a DVD reproduce signal as well as a partial response. As shown in FIG. 2, the vertical axis represents an output gain and the horizontal axis represents a normalized frequency. It is apparent that the optical frequency response allows the output of a signal at a low frequency range and the output is declined by increase of the normalized frequency; almost no output appears at a frequency of more than 0.3. The three curves of partial response (1, 1), (1, 2, 1), and (1, 3, 3, 1) are expressed by the following equations (1), (2), and (3) respectively. Be noted that 1+D denotes addition of a current signal and a delayed-by-1T signal.
1+D=1+exp (xe2x88x92jw)xe2x80x83xe2x80x83(1)
(1+D)2=(1+exp(xe2x88x92jw))2xe2x80x83xe2x80x83(2)
(1+D)3=(1+exp(xe2x88x92jw))3xe2x80x83xe2x80x83(3)
Now considered is a judgment level on the basis of NRZ (non-return to zero) codes. While the judgment levels used for the NRZ codes are two (denoted by 2-level), for the partial response (1, 1) are three (3-level), for the partial response (1, 2, 1) are five (5-level), and for the partial response (1, 3, 3, 1) are nine (9-level). For example, when a signal of xe2x80x9c011111000xe2x80x9d is given at the partial response (1, 1), it is added with a delayed-by-1T signal according to the equation (1) of 1+D and hence expressed by xe2x80x9cx12222100xxe2x80x9d, where x is either 0 or 1 depending on the preceding and succeeding values. The sum is hence expressed by three digits xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d, and xe2x80x9c2xe2x80x9d and the number of judgment levels is three. If the number of judgment levels is increased to a higher number such as five or nine, the judgment may be difficult. Also, the Viterbi circuitry arrangement in a Viterbi decoder may be increased in proportion to the square of a judgment level.
It is understood that even if the partial response (1, 3, 3, 1) is used for eliminating the above problems in any conventional digital reproduced signal processing apparatus, the judgment may be much difficult with the emphasizing at a high frequency range reduced and the overall arrangement of a Viterbi circuit will be bulky. Also, the frequency response of a DVD reproduced signal may largely be dissimilar the partial response (1, 3, 3, 1) and the two will hardly be equalized. It is not desirable to use the partial response (1, 3, 3, 1).
Because of the foregoing problems, a variety of analog reproduced signal processing apparatuses have reluctantly been utilized for reproduction of DVD signals.
It is an object of the present invention for solving the foregoing problems to provide a digital reproduced signal processing apparatus which utilizes a partial response having a frequency characteristic suited for picking up the signal of high-frequency cutoff type based on code words of which the minimum code length is 2T or higher and has a simple circuitry arrangement. According to the present invention, the equalization of the digital reproduced signal with the use of a Finite Impulse Response (referred to as FIR hereinafter) filter employs (a, b, b, a) type of partial response as the target characteristic and the FIR filter comprises a delay elements arrange in which any two delay elements are connected to one multiplier, whereby the overall circuitry arrangement can significantly be reduced. In particular, (3, 4, 4, 3) type of the partial response is used thus providing ease of the equalization of the frequency response of DVD signals.
As a first mode for embodying the present invention, a digital reproduced signal processing apparatus for reading out a signal from a recording medium of a digital recording apparatus of high-frequency cutoff type is provided comprising: a read head for reading out the recorded signal and releasing it as an analog reproduced signal; a low-pass filter for removing high frequency noise from the analog reproduced signal; an analog/digital converter for converting the analog reproduced signal filtered by the low-pass filter to a digital reproduced signal; an FIR filter for filtering the digital reproduced signal with the use of an adaptive equalization coefficient; an adaptive equalization coefficient setting device for equalizing the impulse response of the digital reproduced signal with the impulse characteristic of a partial response defined by (a, b, b, a) and determining the adaptive equalization coefficient to release the digital reproduced signal filtered by the FIR filter as an equalized digital reproduced signal; a phase comparator for detecting a phase error signal from the digital reproduced signal or the equalized digital reproduced signal; a partial response temporal judgment device responsive to the output signal of the FIR filter for producing and feeding a temporal data judgment signal to the adaptive equalization coefficient setting device and the phase comparator; a Viterbi decoder for decoding the equalized digital reproduced signal released from the FIR filter to a data for judgment; a loop filter for filtering the phase error signal detected by the phase comparator; a digital/analog converter for converting the filtered phase error signal to an analog signal; and a voltage-controlled oscillator controlled by the output signal of the digital/analog converter for oscillation to produce and feed a clock signal to the Viterbi decoder. The digital reproduced signal processing apparatus of the first mode is hence capable of performing partial response reproduction, is improved in the reading performance as compared with any conventional analog reproducing apparatus, and advantageous in the down-sizing of the circuitry arrangement.
As a second mode for embodying the present invention, the digital reproduced signal processing apparatus of the first mode is modified in that the FIR filter comprises: a delay elements array having a series of delay elements connected one another in a row for delaying the input signal by 1T at each element; a plurality of multipliers, each arranged for multiplying the equalization coefficient by the input signal or a delay signal released from a node of the delay elements array which stands after at least two or more consecutive delay elements of the delay elements array; and an adder for summing the output signals of the multipliers.
As a third mode for embodying the present invention, the digital reproduced signal processing apparatus of the first mode is modified in that the adaptive equalization coefficient setting device is provided with (a, b, b, a) type of the impulse response or partial response which is a target for equalization and arranged to timely update the adaptive equalization coefficient for varying the values a and b in (a, b, b, a) so that the square mean of a difference between the temporal data judgment signal from the partial response temporal judgment and the equalized digital reproduced signal is minimum, whereby a desired characteristic of the partial response can be obtained.
As a fourth mode for embodying the present invention, the digital reproduced signal processing apparatus of the first mode is modified in that the partial response temporal judgment device has a function of converting the data determined by level judgment with a partial response of lower order or any 2-level detection to a judgment level for the partial response which is used for the adaptive equalization and also a function of temporarily identifying a zero-crossing region.
As a fifth mode for embodying the present invention, the digital reproduced signal processing apparatus of the first mode is modified in that the Viterbi decoder comprises: a branch metric calculator having a means for varying the Viterbi judgment level corresponding to the partial response (a, b, b, a); a path metric calculator responsive to a select signal fed from a control signal generator for accumulating the output signals of the branch metric calculator; the control signal generator for producing the select signal indicative of a path at the highest of the probability of being selected by comparing between the output signal of the path metric calculator and the output signal of the branch metric calculator; and a path memory having state memories, so that the adaptive equalization coefficient setting is compatible with (a, b, b, a) type of the partial response.
As a sixth mode for embodying the present invention, the digital reproduced signal processing apparatus of the third mode is modified in that the adaptive equalization coefficient setting device is arranged in that the period of updating the equalization coefficient is thinned out at intervals of nT (n=1, 2, 3, . . . ).
As a seventh mode for embodying the present invention, the digital reproduced signal processing apparatus of the fifth mode is modified in that the Viterbi decoder is arranged in that the paths limited by the code length are eliminated and the Viterbi judgment level can arbitrarily be modified by changing the values a and b in the partial response (a, b, b, a).
As an eighth mode for embodying the present invention, the digital reproduced signal processing apparatus of the first or third or fifth or seventh mode is modified in that the partial response (a, b, b, a) is (3, 4, 4, 3).
As a ninth mode for embodying the present invention, a digital reproduced signal processing method is provided comprising: a first step of reading out a signal by a read head from a recording medium of a digital recording apparatus of high-frequency cutoff type, filtering the signal read out with the use of a low-pass filter for removing its high frequency noise, and analog-to-digital converting it to a digital reproduced signal; a second step of filtering the digital reproduced signal produced at the first step with the use of an FIR filter controlled by an adaptive equalization coefficient for having an equalized digital reproduced signal; a third step of equalizing the impulse response of the digital reproduced signal with the impulse characteristic of a partial response specified by (a, b, b, a) to determine the adaptive equalization coefficient so that the equalized digital reproduced signal is released as an output signal from the FIR filter; and a fourth step of decoding the equalized digital reproduced signal released from the FIR filter to a data for judgment. This signal processing method is hence improved in the reading performance as compared with any conventional analog reproduced signal processing method and is optimum for realizing an improved digital reproduced signal processing apparatus which is significantly reduced in the circuitry arrangement.
As a tenth mode for embodying the present invention, the digital reproduced signal processing method of the ninth mode is modified in that the third step is featured of equalizing the impulse response of the digital reproduced signal produced at the second step with the impulse characteristic of the partial response specified by (3, 4, 4, 3) to determine the adaptive equalization coefficient for releasing from the FIR filter the equalized digital reproduced signal as an output signal and the fourth step is featured of decoding the equalized digital reproduced signal to the data for judgment in accordance with the partial response (3, 4, 4, 3).